I agree 100% with what Secretgirl said. Once you get to a certain level of beauty, it's basically impossible to go out without someone thinking a nasty thought about you, often times because they're jealous. Clearly her dress shows off her physique, but hers is really good and I do hear that she's one of the women in Hollywood because she works hard for it. Once you've got her body, it's kind of hard to dress in a way that doesn't make it obvious.
It could be because you have a negative mental image of Megan Fox. As far as her personality goes, I know I do... she's kind of got a pretty absurd ego. But you can't really deny that she's very physically attractive. Jessica Biel has a really good body, probably just about the best in Hollywood. She wears dresses that make her physique pretty obvious. There's a picture of her on the red carpet in a white dress with a revealing back that made her butt pretty famous ( link ). And nobody calls her a slut basically because she's a nicer person than Megan Fox is (heh)
So I mean I think it's not really the dress as much as it is Megan Fox. But she does make it look good, but I guess that's what makes it easy to call her (somewhat incorrectly) a "slut", her being a good-looking person that is very dislikeable.